The (optional) spread radius allows to make the shadow bigger without
enlarging the blur value. Mozilla supports this parameter for the
-moz-box-shadow property.
https://bugzilla.gnome.org/show_bug.cgi?id=613832
The idea behind this move is that we have a lot more control over
rendering if StWidget isn't a big pile of actors, and things are
more efficient.
https://bugzilla.gnome.org/show_bug.cgi?id=607500
